1. Field of the Invention
The invention relates to an exhaust emission control system of an internal combustion engine.
2. Description of Related Art
Various types of exhaust emission control systems for purifying exhaust gas emitted from combustion chambers of internal combustion engines are known in the art. For example, Japanese Laid-open Patent Publication No. 6-159037 discloses an exhaust emission control system that includes a particulate filter for trapping particulates contained in exhaust gas and burning and removing the trapped particulates. The particulate filter carries a NOx absorbent that is useful for promoting combustion of the particulates trapped in the filter. In operation, the NOx absorbent supported by the particulate filter absorbs nitrogen oxides (NOx) contained in the exhaust gas when an oxidizing atmosphere exists around the particulate filter, and releases the absorbed NOx when a reducing atmosphere exists around the filter so that the NOx is reduced and removed by a reductant contained in the reducing atmosphere.
When the NOx is reduced and removed by the reductant, reaction heat is generated, and the temperature of the particulate filter is raised due to the reaction heat. Generally, particulates trapped in the particulate filter are more likely to be ignited as the temperature of the particulate filter increases. Namely, in the exhaust emission control system disclosed in the above-identified publication, the NOx absorbent serves to promote combustion of the particulates trapped in the particulate filter. Thus, the conventional exhaust emission control system is provided with various measures or means for improving its emission purification efficiency.